Brooke Shields Finally Focusing on Her Body

It's taken 43 years, but Brooke Shields is concentrating on her body for the first time, she tells PEOPLE.

"I've never been one to actually focus on myself physically because everyone else was doing it for me. So I had this detachment, to be honest, for a very long time," Shields said Monday in New York City, at Maybelline's Studio 450 bash celebrating the season premiere of her NBC series Lipstick Jungle, which returns to the air Sept. 24.

"I'm now just ... I celebrate my strength," said Shields, showing off perfectly toned legs in a sparkly Badgley Mischka minidress.

"I'm not petite," Shields, who stands 6 ft., explained. "I've never been very frail or small, so I've had to find ways to celebrate that. And it's really my strength that I celebrate."

Shields, who bares her stomach on the cover of this month's Shape magazine, says she works out to feel good – not to look good.

"I spin and I do yoga, and that's it ... I can carry my kids and I can dance and my legs get me through Broadway shows. And to me, when they function for me, not just look a certain way, then I feel lucky."